RFC: Simplfying hyperthreading distinctions

[Nathan Whitehorn]

On 03/06/15 12:44, John Baldwin wrote:
> Currently we go out of our way a bit to distinguish Pentium4-era
> hyperthreading from more recent ("modern") hyperthreading.  I suspect that
> this distinction probably results in confusion more than anything else.
> Intel's documentation does not make near as broad a distinction as far as I
> can tell.  Both types of SMT are called hyperthreading in the SDM for example.
> However, we have the astonishing behavior that
> 'machdep.hyperthreading_allowed' only affects "old" hyperthreads, but not
> "new" ones.  We also try to be overly cute in our dmesg output by using HTT
> for "old" hyperthreading, and SMT for "new" hyperthreading.  I propose the
> following changes to simplify things a bit:
>
>    1) Call both "old" and "new" hyperthreading HTT in dmesg.
>
>    2) Change machdep.hyperthreading_allowed to apply to both new and old HTT.
>       However, doing this means a POLA violation in that we would now disable
>       modern HTT by default.  Balanced against re-enabling "old" HTT by default
>       on an increasingly-shrinking pool of old hardware, I think the better
>       approach here would be to also change the default to allow HTT.
>
>    3) Possibly add a different knob (or change the behavior of
>       machdep.hyperthreading_allowed) to still bring up hyperthreads, but leave
>       them out of the default cpuset (set 1).  This would allow those threads
>       to be re-enabled dynamically at runtime by adjusting the mask on set 1.
>       The original htt settings back when 'hyperthreading_allowed' was
>       introduced actually permitted this via by adjusting 'machdep.hlt_cpus' at
>       runtime.
>
> What do people think?

I'm fine with whatever naming, but if we're making new sysctls, 
especially for the cpuset case, is there a reason to hide the behavior 
under machdep? We support at least three non-x86 CPUs with SMT (POWER8, 
Cell, and POWER5) and the relevant scheduling logic should be MI. At 
least POWER8 supports 8 threads per core, so you might also want more 
granularity than just "on" or "off".
-Nathan